The genetics of rheumatoid arthritis suggest that a combination of inherited alleles and environmental factors contribute to the development of arthritis. T cell reactivity is implicated in RA based on the linkage of RA to particular MHC alleles. Our animal model, proteoglycan-induced arthritis (PGIA) has many similarities to RA and so has the potential to provide important information on the mechanism of human disease. Similar to RA, PGIA is critically dependent on auto reactive T cell activation. In PGIA, activation of Th1 cells and the production of IFN3 are critical for development of severe arthritis. In this, the conventional model of PGIA, the route of antigen exposure is intraperitoneal. We demonstrate that altering the route of antigen exposure from intraperitoneal to intradermal induces a Th17 instead of a Th1 T cell response. These findings suggest that antigen presentation changes the early events in T cell polarization and that the innate microenvironment at these different tissue sites may provide unique signals that foster T cell differentiation. Very little is known about how autoimmune responses initiated at different tissue sites might affect disease. The overall goal is to understand the rules governing T cell differentiation at distinct tissue sites. Our strategy in aim 1 is to determine the cytokine environment which contributes to Th cell polarization and to determine if the immunopathology of the resulting arthritis is similar (or different) in Th1- versus Th17-mediated PGIA. In aim 2 and aim 3 we will begin to dissect the mechanism by which the route of antigen exposure contributes to T cell polarization. In aim 2 we will locate the lymphoid tissue sites where T cells are initially primed and produce cytokines and attempt to redirect the differentiating T cell population by altering the cytokine environment. In aim 3 we will identify the antigen presenting cells which drive Th1/Th17 mediated PGIA and determine the cytokines and co-stimulatory molecules involved in T cell polarization. If we can begin to understand how these T cell subsets are initiated we may be able to identify different subtypes of RA which could lead to the development of more effective therapeutic strategies for selective blockade of disease. This is an important and exciting area of investigation, which is expected to shed new light on the mechanisms of CD4+T cell mediated disease in RA, and more generally in autoimmune disease.